Reference frames of at least two remote systems that are not mechanically linked are often required to be aligned. Though the applications are various, in the case of Quantum Key Distribution (QKD) over mobile devices in free space, correct orientation of remote transmitting and receiving systems is vital. QKD is a quantum mechanical means of secret key exchange between two remote parties. The quantum systems (qubits) used in the implementation of QKD are often single particles of light known as photons. The bit values of the secret key to be exchanged are commonly coded within the orientation of the polarization of the photon. A variation or offset in the orientation of the reference frame of the transmitter and receiver would imply mismatched bases between the sender and receiver, resulting in an incorrect measurement of the polarization by the receiver. It is imperative, therefore, that both the local and remote reference frames are locked at identical orientations. Since only deterministic readings are useful, successful qubit distributions achieved with qubits prepared and measured in an identical basis, a misalignment between the intended and actual measurement of the system will induce a probabilistic outcome to the measurement of the qubit. This will result in a portion of the outcomes to reflect the incorrect bit value and, hence, increase Quantum Bit Error Rate (QBER).
Some conventional mobile free-space QKD systems require sophisticated tracking systems in order to keep suitable alignment or uplinks between remote transmitters and/or receivers. These systems are often expensive and make use of Polaroid lenses to align the remote transmitters and/or receivers with each other. In addition, some conventional systems employ feedback loops since power measured at receiver systems depends on the distance between the transmitter and receiver.
It is desired to have a simpler, less expensive and more robust means to align bases of remote systems.